The present invention relates to a method for the preparation of a coated solid medicament form using a cellulose ether as the coating material.
As is well known, most of the solid medicament forms, such as tablets, granules, capsules and the like, in recent years are provided on the surface with a film coating of a polymeric material for the object of protecting the effective ingredients from the influence of the ambient atmosphere as well as imparting an attractive appearance. Various kinds of organic polymers have been used for such a purpose including those soluble in organic solvents and those soluble in water. The use of a polymer soluble in organic solvents is, however, disadvantageous in respect of the expensiveness of the solvent as compared to water as well as the problems of pollution of the environmental atmosphere and adverse influence on the workers' health. Therefore, the technological trend in this matter is toward the use of a water-soluble polymer such as, typically, a water-soluble cellulose ether by directly spraying an aqueous solution of the polymer to the solid medicament forms in a coating machine.
It should be noted that a problem must be solved for the full utilization of water-soluble cellulose ethers as a material for film-coating of solid medicament forms. Namely, the coating solution prepared by dissolving a cellulose ether should have a viscosity not too high to ensure smooth proceeding of the coating procedure. For example, an acceptable coating solution should have a viscosity not much higher than 100 centipoise at the coating temperature when the cellulose ether dissolved therein is a hydroxypropyl methyl cellulose. This limitation in the solution viscosity naturally limits the concentration of the cellulose ether dissolved therein because a solution of higher concentration naturally has a higher viscosity. In this regard, the concentration of the cellulose ether should not exceed 13% by weight or, usually should be in the range from 4 to 12% by weight even when the weight-average degree of polymerization of the hydroxypropyl methyl cellulose is so low as in the range of 65 to 340, of which a 2% by weight aqueous solution has a viscosity of 3 to 15 centipoise at 20 .degree. C. Incidentally, the above mentioned weight-average degree of polymerization of the hydroxypropyl methyl cellulose is not an accurate value because no well established method is known for the determination of the average degree of polymerization or average molecular weight of a water-soluble cellulose ether. Accordingly, it is a generally accepted way in the art to define the average degree of polymerization of a water-soluble cellulose ether product not by the actual value thereof but instead by the viscosity of a 2% by weight aqueous solution of the polymer at 20.degree. C., which is referred to as the 2% solution viscosity hereinbelow.
Although no definite standard can be given to the above mentioned 2% solution viscosity which a water-soluble cellulose ether to be used for the film-coating process of solid medicament forms should have, because it is dependent on various factors including the concentration of the cellulose ether in the coating solution, the upper limit of the viscosity of the coating solution, types of the equipments used for the coating process such as coaters and pumps, types of the solid medicament forms to be coated and so on, a water-soluble cellulose ether exhibiting a 2% solution viscosity of several hundreds centipoise or higher can hardly be used as a coating material of solid medicament forms unless the concentration of the coating solution be decreased to less than the desirable range of 5 to 10% by weight to give a viscosity of 1000 centipoise or lower to the coating solution suitable for the coating works. The use of such an unduly diluted coating solution is of course undesirable since a sufficiently thick coating film cannot be obtained on the solid medicament forms by a single procedure of coating to greatly decrease the efficiency of the coating process.
The use of a water-soluble cellulose ether having a low average degree of polymerization is of course undesirable in respect of the low mechanical properties of the coating film formed thereof on the solid medicament forms while the use of a cellulose ether having a high average degree of polymerization is limited for the above mentioned reason. Accordingly, it is eagerly desired to develop an efficient method for providing a film coating on various solid medicament forms by using a high-polymeric water-soluble cellulose ether as the coating material.